The present invention relates to an ink jet recording apparatus. More specifically, the present invention relates to an ink jet recording apparatus which includes transfer barrels before and after a processing barrel that performs predetermined processing, such as a drying barrel that dries a recording medium such as a cut sheet-shaped recording sheet on which an image is recorded with ink droplets, and which is capable of preventing detection errors of an abnormal pass (a paper jam, namely a jam) of a recording medium by a detection sensor that is disposed in each of the two transfer barrels so as to detect a pass (sheet pass) of the recording medium.
In the related art, an ink jet recording apparatus that performs recording by ejecting ink droplets from nozzle holes of an ink jet head to form an image on a cut sheet-shaped recording sheet is used as a printer.
In such an ink jet recording apparatus, a conveying unit such as a conveying roller conveys recording sheets picked up one by one in a feeding tray from a sheet feeding unit onto a platen of an image forming unit (a recording unit). Ink droplets are ejected imagewise from the nozzle holes of an ink jet head while scanning the ink jet head relative to the recording sheet on the platen to form an image on the recording sheet. The recording sheet on which an image based on the ink droplets is formed is conveyed to a drying unit and is heated and dried. The recording sheet in which the image is fixed is discharged to a discharge tray.
In such an ink jet recording apparatus, since the conveying unit conveys recording sheets picked up one by one, a paper jam, commonly called a jam may occur in various portions, such as, for example, a sheet-pickup unit from the feeding tray, an image forming unit, a drying unit, a discharging unit, and a processing unit that performs pre-processing or post-processing on a recording sheet. When a jam occurs, it is necessary to open the portion where the jam occurs and remove the jammed sheet, for example, by pulling out the recording sheet. In particular, jams occurring in the image forming unit that includes an ink jet head which may cause a problem such as ejection error during image formation and the drying unit which may cause abnormal heating or abnormal temperature rise are critical. Thus, it is necessary to detect a jam accurately and quickly and stop ejection of droplets from the nozzles of the ink jet head and drying in the drying unit.
In particular, in the drying unit, the recording sheet may get burnt or catch fire due to abnormal heating or an abnormal temperature rise, a user may get burned when removing the recording sheet, and constituent elements may be damaged. JP 2001-96727 A proposes an ink drying device which includes an abnormality detecting unit that detects an abnormality, for example, a jam in a drying region (drying unit), and a control unit that stops the driving of a heater in the drying region when an abnormality is detected by the abnormality detecting unit and then drives only a cooling unit for a predetermined period until the temperature of the drying region decreases to a predetermined temperature.
The ink drying device of JP 2001-96727 A uses a jam detecting unit that detects a stay for a predetermined period or longer of an ink-attached recording medium inside the device as well as a temperature detecting unit as the abnormality detecting unit. In the ink drying device of JP 2001-96727 A, sheet detecting sensors that detect the entrance and exiting of a recording sheet are arranged at the input and output ports of the ink drying device (dryer) as the jam detecting unit so as to detect a jam by determining whether the recording sheet stays for a predetermined period or longer based on the period between the entrance and the exiting of the recording sheet.
Moreover, in the recording unit, since it is desirable that the gap between the ink jet head and the recording sheet is as small as possible from the perspective of image quality, the gap is set to be very small taking a bending state and a floating of the recording sheet into consideration. Therefore, when a jam occurs, and the portion where the jam occurs is opened, sheet powder, dust, or the like may enter into the nozzles of the ink jet head. Moreover, when removing by pulling out the recording sheet, the ink jet head may be damaged by the recording sheet contacting therewith, and the nozzles may cause ejection errors. Furthermore, the recording sheet may be torn to pieces by being caught at the ink jet head or other members, and the torn pieces remain there, which may cause another jam or other problems such as a failure in various portions.
Therefore, JP 2007-144633 A proposes an ink jet recording apparatus which includes a jam detecting unit that detects a jam of a recording sheet and a gap varying unit that varies the gap between an ink jet head and a platen based on the results of the detection by the jam detecting unit.
By doing so, the ink jet recording apparatus of JP 2007-144633 A makes it easy to remove the jammed recording sheet when a jam is detected by increasing the gap between the ink jet head and the platen when removing the jammed recording sheet. In this way, the recording sheet is prevented from making contact with the ink jet head.
In the ink jet recording apparatus of JP 2007-144633 A, an entrance sensor provided on the entrance side on the conveyance path of a recording sheet, a registration sensor provided right before a print region, an exit sensor provided on the exit side, and a timer are used as the jam detecting unit. The ink jet recording apparatus calculates the period taken to convey the recording sheet by the distance between the entrance sensor and the registration sensor at a predetermined speed, calculates the period taken to convey the recording sheet by the distance between the registration sensor and the exit sensor in accordance with printing conditions such as the length in the longitudinal direction of the recording sheet, an image-quality mode, and the quantity of print data, and calculates a total conveyance period obtained by adding both periods. When a front sensor such as the entrance sensor or the registration sensor detects the leading end or the trailing end of the recording sheet, or when a print timing occurs, the timer is started. It is determined that a jam has occurred when the rear sensor was unable to detect the leading end or the trailing end of the recording sheet after the conveyance period corresponding to the distance between two sensors has elapsed. Moreover, it is determined that a jam has occurred when one sensor which has detected the leading end of the recording sheet was unable to detect the trailing end after the elapse of the time taken for the leading end and the trailing end of the recording sheet to pass through the sensor.